The traditional method to compare differently stabilized thermoplastics pipes and to obtain information for pipe design consists of stress rupture experiments with pipes under constant internal pressure. The tests are conducted under specified conditions of internal and external environment (usually water or air), at various pressure levels and frequently at several temperatures. Usually the stress rupture curve of plastics-pipes (hoop stress versus failure time) can be divided into three regimes. Region Ⅰ, at high stress levels, is characterized by a mechanically induced ductile failure, in Region Ⅱ, at intermediate loads and lifetimes, mechanical-chemical degradation is responsible for the initiation and propagation of a single crack with little plastic deformation. Finally in Region Ⅲ, at very low stress levels and thus long lifetimes, chemical aging leads to global polymer degradation and brittle failure. Based upon the observation, that failure in Region Ⅱ of the stress rupture curve, which is of utmost practical importance, is governed by a two stage process consisting of a crack initiation stage and a period of stable, creep crack growth (CCG), linear elastic fracture mechanics (LEFM) has been recently applied by several authors to characterize the CCG resistance also in thermoplastics pipes. As this failure Region Ⅰs known to be influenced by the type and content of the stabilizer or stabilizer system used, it appears meaningful, to also study the effect of stabilizers on the CCG behavior of pipe materials using LEFM methods, and to correlate CCG data with the results on life times of pressurized pipes
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